FIG. 2(a) is a cross-sectional view showing a conventional heterojunction avalanche transistor. FIG. 2(b) shows collector current vs. collector voltage characteristics with a parameter of base current in a common-emitter configuration. FIG. 2(c) shows collector current vs. collector voltage characteristics with a parameter of base voltage in a common-emitter configuration. In FIG. 2, reference numeral 1 designates a semi-insulating InP substrate. An n.sup.+ type InGaAsP contact layer 2 is disposed on the entire surface of the substrate 1. An n type InP emitter layer 3 is disposed on the center portion of the contact layer 2. A p type InGaAsP base layer 4 is disposed on the entire surface of the emitter layer 3. An n type InP collector layer 5 is disposed on the center portion of the base layer 4. A collector electrode 6 is connected with the collector layer 5 via an n.sup.+ type InGaAsP contact layer 2A. A base electrode 7 is connected with the base layer 4 via a Zn diffusion region 9 produced on the periphery of the base layer 4. An emitter electrode 8 is connected with the emitter layer 3 via the contact layer 2.
Collector current vs. collector voltage characteristics as a parameter of base current in a common-emitter configuration will be described with reference to FIG. 2(b). In a bipolar transistor, collector current I.sub.C is represented as follows: ##EQU1## where I.sub.B is base current, M is carrier multiplication factor in the base-collector junction, .alpha. is base transport factor, and .gamma. is emitter injection efficiency. In the conventional heterojunction avalanche transistor, since the energy band gap of the emitter layer is larger than that of the base layer, the emitter injection efficiency .gamma. can be regarded as 1. When the collector current I.sub.C is increased, the collector voltage is mainly applied to the emitter-base junction when the collector current I.sub.C is low. However, once the resistance of the emitter-base junction decreases, the emitter-base voltage scarcely increases further and the current at this junction becomes a constant voltage source. Therefore, when the collector current I.sub.C is further increased, the collector voltage is applied to the base-collector junction, which leads to the increase of the reverse bias at the base-collector junction. The base transport factor .alpha. approaches 1 as the collector current I.sub.C increases, and the carrier multiplication factor M increases as the base-collector reverse bias increases. Then, as the collector voltage increases, M.alpha..gamma. of the equation (1) approaches 1, whereby the collector current I.sub.C drastically increases. When the collector current is further increased, M.alpha..gamma. may exceed 1. However, M.alpha..gamma. cannot exceed 1 from the equation (1). Since the base transport factor .alpha. approaches to 1 in accordance with the increase in the collector current I.sub.C, the carrier multiplication factor M decreases to keep M.alpha..gamma. below 1. The decrease in the carrier multiplication factor M means a decrease in the collector voltage. Then, S-shaped negative differential resistance, which means that the collector voltage decreases in accordance with an increase in the collector current I.sub.C, is obtained (base currents represented by A in FIG. 2(b)).
At some value of base current, no S-shaped negative differential resistance is obtained, but the collector current drastically increases in accordance with the increase in the collector voltage when M.alpha..gamma. approaches 1 but does not exceed 1 (base currents represented by B in FIG. 2(b)).
Next, collector current vs. collector voltage characteristics with a parameter of base voltage in a common-emitter configuration will be described with reference to FIG. 2(c). When the collector voltage is lower than the base voltage, since both of the emitter-base junction and the base-collector junction are forward biased and the transistor is in a saturated region, the collector current increases in accordance with the increase in the collector voltage. When the collector voltage is further increased, the base-collector junction is reverse biased. Therefore, the collector current is approximately determined by the base voltage. When the collector voltage is further increased, the reverse bias of the base-collector junction increases, and as a result the carrier multiplication factor M increases and the term of M.alpha..gamma. in the equation (1) approaches 1. Then, similarly as the situation described with reference to FIG. 2(b), S-shaped negative differential resistance appears at the base voltage where M.alpha..gamma. is about to exceed 1, and the collector current drastically increases against the collector voltage at the base voltage where M.alpha..gamma. approaches 1 but does not exceed 1.
In such a conventional heterojunction avalanche transistor, since the base transport factor .alpha. approaches 1 in accordance with an increase in the collector current, B the S-shaped negative differential resistance characteristic is only obtained in both cases by taking the base current and taking the base voltage as a parameter, respectively, in the common-emitter configuration. Therefore, when a memory circuit is constituted using such a heterojunction avalanche transistor, a bistable memory element is required in addition to the heterojunction avalanche transistor as a trigger element.